1. Field of the Invention
The present invention relates to a superconducting resistive current limiter, particularly to a high-temperature superconducting resistive current limiter.
2. Description of the Related Art
Superconducting fault current limiters are devices that automatically limit fault current in high voltage networks to a low current value close to the nominal current. The benefit of such a device is that it reduces drastically the short circuit power of the high voltage network and, thus, allows to interconnect networks without increasing the short circuit power or to decrease safety margins so that other machineries connected to the network can be designed for lower short circuit power and, therefore, can be made lighter and cheaper.
Superconductors, in particular, high-temperature superconductors, offer a great potential as resistive fault current limiters which enable rapid and effective current limitation, automatic recovery, negligible impedance during normal operation and application at high voltage.
Current limiters based on high-temperature superconducting materials make use of the property of superconducting materials to switch from there superconducting state with essentially no resistance to a normal conductive and normal resistive state when at least one of its critical temperature (Tc), critical magnetic field (Hc) or critical current (Ic) is exceeded.
For example, in case of fault the current flowing through the superconductor material exceeds the critical current of the superconductor material due to large surge current and the superconductor material undergoes transition from the superconducting state to the normal conducting state. This transition is also termed “quenching”.
Typically, a superconducting fault current limiter comprises one or more superconducting limiter elements housed in an insulation housing such as a cryostat filled with a cooling medium for cooling the superconducting limiter elements below their critical temperature Tc at which they exhibit superconducting properties.
The critical temperature depends on the specific superconductor material. Suitable cooling mediums are for example nitrogen, helium, neon, hydrogen or mixtures thereof in their liquid state.
High-temperature superconductors within the meaning of the present invention are those having a critical temperature of 30 K or more, preferably of 77 K or more. Preferred are high-temperature superconductors with a critical temperature of 77 K or more since for cooling liquid nitrogen can be used which is easy to handle and cheap.
High-temperature superconductors with a critical temperature of 77 K or more are, for example, those known as high-temperature superconducting ceramic oxides.
Preferably, the high-temperature superconductor ceramic oxide is selected from the group consisting of bismuth-based, thallium-based, rare earth metal based, and mercury-based superconductor ceramic oxides.
Typical examples thereof comprise ceramic oxide superconductors based on Bi-Ae—Cu—O, (Bi, Pb)-Ae—Cu—Oy, Re-Ae—Cu—Oy, Tl-Ae—Cu—Oy or Hg-Ae—Cu—Oy wherein y represents the relative oxygen content in the range appropriate for the particular superconductor material to have superconducting property.
In the above formula Ae means at least one alkaline earth element, particularly, Ba, Ca and/or Sr.
Re means at least one rare earth element, particularly Y or a combination of two or more of the elements Y, La, Lu, Sc, Sm, Nd or Yb.
There are known fault current limiters comprising bulk limiter elements made of high-temperature superconductor material in form of tubes, rods, cylindrical coils etc. Such bulk limiter elements can be obtained by, for example, melt-casting process as disclosed in EP 1 876 683 A1, EP 1 524 748 B1 and EP 1 622 210 B1.
Apart from high-temperature superconductor bulk limiter elements there are known applications using thin films made of high-temperature superconductor material.
Thin films made of high-temperature superconductor materials are used in a form referred to “coated conductor”.
Generally, coated conductors are tape-like conductors of long length having a multilayer composition with a substrate, a superconductor layer and a varying number of buffer layers between the substrate and the superconductor layer. The buffer layer(s) serve to compensate for the various different properties of the materials used.
A high-temperature superconducting resistive fault current limiter making use of a coated conductor is disclosed in US 2008/0070788 A1. In the design disclosed a coated conductor tape is used which is wound to a flat, disc-shaped bifilar coil, with a distance through which a cooling medium can flow, being maintained between adjacent coil turns. Further, a spacer is provided running in parallel to the coil turns within the space between adjacent coil turns.
In order to be suitable in the production of a fault current limiter a high-temperature superconductor fault current limiter element should meet several requirements.
A basic problem is the way of winding of large length of a coated conductor in order to obtain a functioning high-temperature superconductor fault current limiter element. In doing so a number of requirements must be met which are partially conflicting:
The winding should be space saving and compact. However, the bending radii must not be too small for avoiding mechanical damage of the coated conductor.
The coated conductor should be in direct contact with the cooling medium to allow rapid recovery to the operational state by means of cooling after fault event. This is important since a current limiter should be ready for operation after fault event as soon as possible for accepting nominal power and being ready for the next short circuit event.
The configuration of winding of the coated conductor should have an inductivity as low as possible. Since the fields induced by inductivity cause strong Lorentz forces and decrease the current carrying capacity, in order to compensate these disadvantages the design of the fault current limiter becomes more complicated.
Further, a standardized modular design is desirable for allowing easy adaptation of the fault current limiter to the specific requirements of an electrical application in view of nominal voltage and/or nominal current.
Considering that fault current limiters are operated at high voltage to very high voltage such as 10 kV to more than 100 kV a high breakthrough strength is mandatory which allows the fault current limiter to withstand high overloadings. Nitrogen, which conventionally is used as the cooling medium, has a high breakthrough strength in its liquid state which, however, decreases significantly in the gaseous state. Thus, on heating, in particular during quench, when the liquid nitrogen is evaporated and gas bubbles are generated, the breakthrough strength is significantly lowered. Consequently, the design of the fault current limiter must be such, that partial discharge and flashover are suppressed even when the breakthrough strength of the cooling medium is decreased, for example due to formation of gas bubbles.
Though the design of the bifilar coil disclosed in EP 1 042 820 B1 referred to above essentially meets the requirements of space saving, cooling, low inductivity and standardization there is improvement required with respect to breakthrough strength.
In the bifilar coil design the end (entry) with the lowest voltage level and the end (exit) with the highest voltage level are located next to each other which results in a design of low breakthrough strength and enhanced risk of flashover as illustrated below.
Considering a bifilar coil design according to EP 1 042 820 B1 with a coated conductor length of 50 m and an electrical field strength of 1 V/cm a voltage difference as high as 5000 kV would result between entry and exit, which in case of incoming surge would be still increased. However the larger the voltage difference between two adjacent parts the higher the risk of flashover. Further, lightning surges frequently induces short circuit and there is a need for a safe and reliable protection against such occurrences.
Consequently, there is a need for a fault current limiter having a high breakthrough strength, which can withstand high overloading and safely protects an electrical application against high voltage peaks.
In particular, there is a need for such a fault current limiter which can be safely operated at high to very high voltages.
According to the present invention a high-temperature superconductor (hts) fault current limiter element suitable for a fault current limiter is provided comprising at least one coated conductor of long length and a mandrel tube wherein the at least one coated conductor is helically wound around the mandrel tube in longitudinal direction of the mandrel tube.
In particular, the hts fault current limiter element of the present invention is suitable and designed for use in a stand-alone fault current limiter unit.
Further, the present invention relates to a high-temperature superconductor fault current limiter comprising at least one high-temperature superconductor fault current limiter element according to the present invention.
Preferably, the high-temperature superconductor fault current limiter comprises at least two high-temperature superconductor fault current limiter elements of the present invention which are connected electrically in parallel and/or in series.
In the design of the present hts fault current limiter element current entry is at one end of the mandrel tube and current exit at the opposite end. Consequently, entry and exit are separated from each other by the whole length extension of the mandrel tube. Further, voltage difference between adjacent windings of the coated conductor helically wound onto the mandrel tube, is only small. Thus, a design of high breakthrough strength is obtained, which can withstand high overloadings and is able to safely protect a power application operated at high to very high voltages in fault event.
Moreover the design of the present hts fault current limiter element allows a high degree of standardization and provides modular configuration of a fault current limiter.
The hts fault current limiter of the present invention is a distinct unit which is connected electrically in series with an electrical network for protecting the electrical network against short circuit occurrences.
In particular, the its fault current limiter of the present invention is designed and suitable as a stand-alone fault current limiter.
Due to the modular standardized design of hts fault current limiter elements the hts fault current limiter unit of the present invention can be easily adapted to the requirements of the power application to which it is connected, and can be scaled up or down according to need by varying the number of hts fault current elements. The modular and standardized design of the hts fault current limiter elements of the present invention allow a compact and space saving layout of the fault current limiter unit of the present invention.
Coated conductors and their production processes are well known in the art and are broadly described in the relevant literature as referred to, for example, above.
Coated conductors have a tape-shape of long length, wherein the longitudinal extension significantly exceeds the width or diameter. Generally, they are composed of a substrate, preferably made of a metal which preferably is biaxially textured, an active layer of high-temperature superconductor material and a varying number of buffer layers between the substrate and the superconductor layer.
For example, in a typical coated conductor the substrate can be a flat tape of long length with plane upper and lower surface. A typical width of such a coated conductor tape is from a few millimeters to a few centimeters. According to need the width can be also outside this range.
The coated conductor tape can have an essentially rectangular or essentially round cross-sectional shape. “Essentially rectangular” or “essentially round” means that the cross-sectional shape may deviate from an ideal rectangular or round shape, for example trapezoid or oval.
The substrate can be coated on one side only or on both sides.
Further, in case of an essentially round coated conductor, typically the layers and substrate are coaxially arranged.
Though not restricted thereto currently the rare earth barium cuprate type superconductors of the formula ReBa2Cu3O7-x are conventionally used in the production of coated conductors. Re means at least one rare earth element, particularly, Y or a combination of two or more of the elements Y, La, Lu, Sc, Sm, Nd or Yb, and x represents the relative oxygen content in the range appropriate for the particular superconductor material to have superconducting property.
A particular member thereof is that one known by the reference YBCO-123 wherein the numerical combination 123 stands for the stoichiometric ratio of the elements Y, Ba and Cu.
It is also possible to use other hts materials, for example as referred to initially.
Processes for a production of such coated conductor tapes are well-known in the art and are disclosed, for example, in EP 2 012 373 A1, EP 1 892 319 A1, EP 1 655 788 B1, EP 2 012 373 A1, and EP 09 305 754.5 which are incorporated herein by reference.
The mandrel tube, typically, has a cylindrical shape, preferably with round, oval or polygonal cross-section. The mandrel tube can be a solid body or hollow body. Preferably the mandrel tube is a hollow body which allows cooling medium to be pathed through the channel present in the mandrel tube.
Mandrel tubes suitable for the present invention can be advantageously obtained by well-known injection molding techniques.
The mandrel tube is made of an electrically insulating material such as plastic or fiber glass or of a material of only poor electrical conductivity, for example of stainless steel. Further, preferably the mandrel tube is a rigid component of only low flexibility.
The coated conductor tape is wound helically around the mandrel tube in longitudinal direction from one end portion of the mandrel tube to the opposite end portion.
To be suitable for a compact fault current limiter unit the length of coated conductor tape should exceed the height of the mandrel tube by a significant factor, which means that the length of lay should be only small.
The term “length of lay” means the distance required by one single coated conductor tape to be turned 360° C.
It is also possible to wind two or more coated conductor taps helically in parallel onto the mandrel tube.
In this case the individual coated conductor tapes can be connected electrically in parallel.
The diameter of the mandrel tube should be sufficiently large to allow bending radii at which the coated conductor tapes are not mechanically damaged. Suitable bending radii are typically in the range of 20 mm to 50 mm.
A typical mandrel tube of round cross-section suitable for the present invention can have an overall diameter of about 10 cm to about 50 cm and a length of 50 cm to 150 cm, preferably 60 cm to 120 cm. For example, a typical number of windings per layer of conductor tapes can be 40 to 70 windings per meter of the mandrel tube.
In the following some concrete values are given for a current limiter element suitable for the present invention for a better understanding but without being limited thereto.
Mandrel Tube:
                diameter: 10 cm        length: 110 cmCoated Conductor Tape:        width: mm        nominal current: 100 A (77 K)first embodiment: 1 tape having a length of 50 msecond embodiment: 10 tapes each having a length of 5 meter        
In the first embodiment the nominal current of the fault current limiter element is 100 A at a voltage of 5000 V.
In the second embodiment the nominal current is about 1000 A with a voltage of about 500 V.
A plurality of such modular current limiter elements can be used to construct a current limiter device of modular design. By means of a parallel connection of a number n of such elements with nominal current i and nominal voltage u, a current limiter device for a specific nominal current n·i, and by series connection of a number m of such elements for nominal voltage m·u can be obtained.
That is, using the hts fault current limiter elements of the present invention provides a high degree of freedom for the construction of a current limiter device suitable for save protection of power networks of different requirements with respect to nominal current and nominal voltage.